In early proposals (see for example U.K. Pat. Nos. 855,107 and 869,865), a titanium electrode with a coating of platinum group metal was provided with an inert barrier layer of titanium oxide in the porous places of the coating, this barrier layer preferably being formed or reinforced by a heat treatment. Later, in U.K. Pat. No. 925,080, the inert barrier layer of titanium oxide was preformed by electrolytically treating or heating the titanium substrate in an oxidizing atmosphere prior to application of the platinum group metal. The preforming of such a barrier layer was also advocated in U.K. Pat. No. 1,147,422 with a view to improving the anchorage of an active coating consisting of or containing platinum group metal oxides.
Later, the development of coatings formed of mixed crystals or solid solutions of co-deposited oxides of film-forming metals and platinum group metals (see U.S. Pat. No. 3,632,498) provided commercially viable electrodes which revolutionized the chlor-alkali industry and have become widely used in other applications. With these electrodes, excellent performance was achieved without the need for a reinforced or preformed inert barrier or anchorage layer on the substrate and today it is generally accepted that the preformed or reinforced inert barrier layers are detrimental to performance. In retrospect, the early proposals for preformed or reinforced inert barrier layers appear to have been unsuccessful attempts to avoid defects which were inherent in the previous coatings rather than in the substrate.
Nevertheless, some proposals attempting to improve inert barrier layers have still been made, for example by applying a titanium oxide barrier layer from a solution containing Ti.sup.4+ ions. Again, this has been found to impair performance of the electrodes.
Another approach has been to provide a non-passivating barrier layer or intermediate layer underlying the active outer coating. Typical suggestions have been doped tin dioxide sub-layers; thin sub-layers of one or more platinum metals such as a platinum-iridium alloy; sub-layers of cobalt oxide or lead oxide, and so forth. Although various patents have claimed marginal improvements for these electrodes in specific applications, in practice none of these suggestions has led to any significant improvement or any widespread commercial use.